This invention is generally directed to processes for the preparation of polycrystalline materials. More specifically, the present invention is directed to nucleation methods for the formation of polycrystalline films which films are useful in thermal transfer printing processes as protective coatings, and for heat conductive coatings in microelectronic circuits. In one embodiment of the present invention, there are provided processes for the preparation of polycrystalline diamond films wherein a suspension of diamond powder is applied to a substrate, and subsequently a mixture of gases is decomposed while heating the substrate, thereby enabling the nucleation of the appropriate decomposed gases. Thus, for example, there is initially applied to a substrate, such as a silicon wafer, diamond powder with certain parameters by spin coating, dip coating or spray coating methods, which powder functions primarily as a seeding, or nucleation source of the condensable vapor resulting from the decomposed gases. More specifically, in one embodiment of the present invention subsequent to heating the substrate as illustrated herein, there is introduced into a vacuum chamber gases or mixtures thereof. These gases serve as a source of carbon, and are decomposed by heating at high temperatures, for example 2,000.degree. C. with a filament, or by plasma decomposition, thereby enabling the decomposed gases to deposit on the aforementioned substrate and permitting nucleation, resulting in the formation of the desired continuous polycrystalline diamond films. The process of the present invention has numerous advantages including the rapid growth of high purity polycrystalline diamond films; the avoidance of abrading the substrate as is the situation with many prior art processes; controlled nucleation thereby resulting in the growth of continuous films; and reproducible results.
Present methods for obtaining polycrystalline diamond films are believed to be inadeqate since the surfaces upon which these films are deposited require mechanical abrasion treatments, and this abrasion damages the surface of the substrate in that material is randomly removed therefrom, thus adversely affecting the optical transmission properties and otherwise damaging the structure of this layer which might also contain microelectronic devices. Further, with these processes undesirable nonuniform diamond films result, and such processes are usually irreproducible. More specifically, there is disclosed in the prior art the difficulties of diamond film nucleation by a method using mechanical abrasion of the substrate surface to cause diamond crystals to nucleate and grow into a continuous film. Thus, for example, there is illustrated in the Journal of Applied Physics 63 (1988) 1744, C. P. Chang, D. L. Flamm, D. E. Ibbotson, and J. A. Mucha a variety of techniques for attempting to nucleate diamond. Substrate surfaces were roughened using four different kinds of plasma etches as well as sputtering with no success. Also, with the aforementioned methods the nucleation of diamond was not improved by overcoating a silicon wafer with amorphous carbon, amorphous silicon carbide, photoresist, or very rough textured polysilicon. Further, in a paper by Y. Mitsuda, Y. Kojima and T. Yoshida, Journal of Material Science 22 (1987) 1557, there is described methods for the abrasion of a silicon surface to permit a sufficient density of nucleation sites and enabling growth into a continuous film by mechanically shaking a silicon wafer for one hour in #1000 diamond powder, which was apparently superior to hand polishing with diamond paste. Electron microscopy revealed that the resulting surface roughness (mean amplitude) was about 10 nanometers. The aforementioned disadvantages are avoided with the processes of the present invention.
There is disclosed in Japanese Patent application abstract No. 2138395/Jun. 1987 a process wherein there is applied to substrates diamond particles at low seeding rates, that is 100,000 particles per square centimeter up to 1,000,000 particles per square centimeter, thus it is believed that continuous diamond films are not generated as is the situation with the process of the present invention wherein high seeding rates are selected. More specifically, this abstract indicates that a gas mixture of hydrocarbon and hydrogen is introduced onto the surface heated to 500.degree. to 1,300.degree. C., and diamond is deposited on the substrate by the pyrolysis of the hydrocarbon. Also, with the process disclosed in this abstract it is indicated that the substrate surface has fine particles with SP3 bonds uniformly scattered; the diamond or other particles were applied as suspensions in methanol; and a dense diamond film with uniform thickness was obtained.
In Canadian Patent No. 628,567 and the article Growth of Diamond Seed Crystals by Vapor Deposition, Case Western Reserve University, Volume 19, Number 6, Page 2915, May 1968, Angus et al., there are disclosed methods for growing diamond on seed crystals. Also of similar relevance are U.S. Pat. Nos. 4707,384, the references listed on page 1 of the '384 patent; 3,961,103; 4,060,660; 4,434,188, gas mixture of hydrocarbon and hydrogen selected; 4,490,229; and 4,504,519; Japanese No. 62-133068; and Nature, Vol. 248, Apr. 12, 1974, pages 582 to 584.